Exclude examples/sim_simple.cpp if UMFPACK is unavailable

Not all systems provide UMFPACK so even if Eigen provides the required
bindings, we should not assume that the underlying library and/or
headers are available.  Currently, only examples/sim_simple.cpp uses
UMFPACK unconditionally, so the simplest solution is to exclude that
example unless UMFPACK is available.
do use eigen's umfpack support anymore

use BiCGStab instead. it might be slower, but at least it is always
available...
Merge branch 'master' into optional-umfpack

Conflicts:
	CMakeLists.txt

Import akva2's modified Eigen support.
Fix mismerge in commit b54df70.

We lost the SuiteSparse detection.
Merge pull request #15 from bska/optional-umfpack

Exclude examples/sim_simple.cpp if UMFPACK is unavailable
Merge remote-tracking branch 'andlaus/no-umfpack' into optional-umfpack
Use UMFPACK solver if available in sim_simple.cpp.

Fall back to Eigen's BiCGSTAB solver if UMFPACK is not available.  The
BiCGSTAB is built into Eigen and consequently always available when
Eigen is found.
Merge pull request #17 from bska/optional-umfpack

Use UMPACK only if available